DESCRIPTION: Core binding factor (CBF) was originally identified as a DNA-binding protein that specifically binds to the asymmetric sequence PyGPyGGT, corresponding to the highly conserved "core" site in mammalian type C retrovirus enhancers. CBF binding sites have subsequently been identified in a number of T cell specific genes, providing evidence for the role of CBF as a T-cell transcription factor. Additional evidence for the importance of CBF has come from knockouts of two of the four CBF genes in mice which were shown to be embryonic lethal and to have a profound blockage in hematopoietic development. Isolation and subsequent cloning of CBF showed the protein to be a heteromer consisting of a CBFA subunit and a CBFB subunit. The CBFA subunit contacts the DNA directly, whereas the CBFB subunit does not, as indicated by the lack of any changes in the number of phosphate contacts made by CBFA in the presence of CBFB. Binding of CBFB to CBFA increases the affinity of CBFA for DNA without altering the sequence specificity. The structures of the heterodimerization of CBFB and the DNA-binding domain, or Runt domain, of CBFA have been described by this lab. The mechanism by which CBFB enhances the binding of CBFA has not been established, thus specific aim #1 of this proposal is the determination of the structure of the heterodimerization domain of CBFB in the ternary complex with the Runt domain and DNA. This is part of Dr. Bushweller's overall goal of solving the structure of the ternary complex containing CBFB, CBFA, and DNA. Two of the four genes encoding CBF subunits are proto-oncogenes commonly activated in human leukemias. The inversion and translocations identified in these genes are associated with 30% of de novo acute myeloid leukemias in humans. The inv(16) involving the gene coding for CBFB produces a novel fusion protein with the functional domain of CBFB fused to the coiled-coil domain of a smooth muscle myosin protein. The mechanism of dysregulation caused by this oncoprotein has not been completely elucidated. In order to provide a structural basis for understanding the functioning of this oncoprotein form of CBFB, specific aim #2 proposes to solve the structure of a functional portion of the CBFB-SMMHC oncoprotein.